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Mineralogical signatures of weathered Late Holocene ash layer from the island Mljet in Croatia

Mljet is the southeasternmost pearl of the Croatian archipelago. Due to the prevailing Mesozoic carbonate rocks exposed at the surface, the morphology of the island is characterized by deep dolines and karst valleys. These karstic forms retain fresh water forming shallow lakes (blata) in the NW part of the island. In the SE part, these depressions may be filled by thick aeolian deposits of fine- to medium-grained sand, which was commercially exploited in the past. The Pinjevica pit is the largest excavation on the island, containing estimated amount of 1.4 mega tons of quartz-dominated sand interlayered by 35-40 cm thick vitric tephra (KRKALO & PENCINGER, 1995). The tephra layer is slightly lithified and marked by up to 1 cm wide bioturbation channels with loose sandy infillings. The sands are interpreted as distal Late Würm fluvial deposits that were recurrently transported by wind to the recent position (BOGNAR et al., 1995). Samples were scraped off from the tephra layer in three equal intervals to represent the entire layer. The aliquots were wet sieved and fractions <32 µ ; ; ; m was separated from several larger grain fractions which were separated to heavy and light fractions. In all samples the <32 µ ; ; ; m fraction makes around 98 wt% of the bulk sample. The XRPD analyze of the air dried, etilenglycole saturated and heated <32 µ ; ; ; m fraction revealed only XRD “ amorphous” phase(s). Due to the absence of carbon and volcanic glass in the samples, the phase(s) may be recognized as non-crystalline to poorly-ordered clay mineral(s), allophane and/or imogolite. Allophane and imogolite are indicative for early-stage weathering product of vitric volcanic ash in highly moist or wet systems with good drainage (WADA, 1989) suggesting that similar climatic and pedogenetic conditions were already established in Pinjevica at the time of tephra deposition. Altered tephras can be correlated to coeval distal lavas by their crystaloclastic mineral chemistry. The grain morphology and chemical composition of residual minerals from the analysed tephra allow us to distinguish the volcanic crystaloclasts [sanidine (Or51.5-86.4Ab11.7-45.2An0.8-5.6), plagioclase (Or5.1-9.4Ab38.6-53.2An38.0-56.2), biotite (Mg# = 57.4-63.4 ; TiO2 = 4.4-4.8 wt.%), salite (Di46.9-49.4En34.7-38.6Fs13.1-17.9), ferrosilite (Di1.6-2.4En59.6-63.2Fs35.3-38.8) and Ti-magnetite (8.7 wt.% TiO2)] from the aeolian crystaloclasts (quartz > spinel, diopside/augite, enstatite, actinolite, pumpellyite, epidote, adularia, albite). Spinel composition (Mg# = 32.5-64.0, Cr# = 0.40-0.84) is similar to the spinels from the sandstones of Upper Creataceous to Late Eocene flysch basins located around 450 km to the NW (LENAZ et al., 2000) and from the matrix of Upper Jurassic mélange from the Dinaride Ophiolite Belt in the central Bosnia (MIKES, unpublished). These findings confirm flysch as potential source of Pinjevica sands as it was constrained by BOGNAR et al. (1995). In the adjacent Saplunara Bay a well developed “ pedologic complex” shows onset of soil formation from 3.776-3.363 ka BC to around 3.115 ka BP as may be inferred from the 14C corrected age on Gastropod shells at the bottom of the complex (BOGNAR et al., 1992) and from the predominance of arboreal pollen, particularly of Pinus which is supposed to has been introduced on Mljet by Greek or Roman settlers (JAHNS & VAN DEN BOGAARD, 1998). Since the soil formation interferes with the deposition of analysed tephra, we search for the south Italian volcanoes erupted between around 5.8 and 3.0 ka BP with NE dispersed tephras. The recurrent eruptions from the Campi Flegrei caldera in the period from 4.800 to 3.800 ka BP with the highest magnitude eruption of Agnano-Monte Spina (AMS) dated to 4.100 ka BP (DE VITA et al., 1999) are the best source candidate of the Pinjevica tephra. However, abundant volcanic plagioclase and ferrosilite which are not related to the AMS tephra signalize an another still unidentified source. References BOGNAR, A., KLEIN, V., MESIĆ, I., CULIBERG, M., BOGUNOVIĆ, M., SARKOTIĆ-ŠLAT, M. & HORVATINČIĆ, N. (1992): Quaternary sands at south-eastern part of the Mljet island.- Proc. Internat. symp. Geomorphology and Sea, Mali Lošinj, September 22-26, 99-110, Zagreb. DE VITA, S., ORSI, G., CIVETTA, L., CARANDENTE, A., D’ ANTONIO, M., DEINO, A., DI CESARE, T., DI VITO, M.A., FISHER, R.V., ISAIA, R., MAROTTA, E., NECCO, A., ORT, M., PAPPALARDO, L., PIOCHI, M. & SOUTHON, J. (1999): The Agnano-Monte Spina eruption (4100 years BP) in the restless Campi Flegrei caldera (Italy).- J. Volcanol. Geotherm. Res., 91, 269-301. JAHNS, S. & VAN DEN BOGAARD, C. (1998). New palynological and tephrostratigraphical investigations of two salt lagoons on the island of Mljet, south Dalmatia, Croatia.- Veget. Hist. Archaeobot., 7, 219-234. KRKALO, E. & PENCINGER, V. (1995): Mineralne sirovine otoka Mljeta.- Ekološke monografije, 6, 147-154, Zagreb. LENAZ, D., KAMENETSKY, V.S., CRAWFORD, A.J. & PRINCIVALLE, F. (2000): Melt inclusions in detrital spinel from SE Alps (Italy-Slovenia): A new approach to provenance studies of sedimentary basins.- Contrib. Mineral. Petrol., 139, 748– 758. WADA, K. (1989): Allophane and imogolite.- In: DIXON, J.B. & WEED, S.B (eds.): Minerals in Soil Environments. Soi. Sci. Soc. of Amer., Madison, WI, 1051-1087.

mineralogy; vulcanic ash; Mljet; Croatia

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